Temperature dependent anisotropy of Cu(2) nuclear relaxation rate in YBa 2Cu 3O 7 below T c

Abstract

We have measured the anisotropy of the planar Cu nuclear spin-lattice relaxation rate in YBa 2Cu 3O 7 in a small magnetic field (0.44 T). The anisotropy is almost temperature independent in the normal state but decreases rapidly below T c, shows a minimum near 70 to 80 K and increases again at lower temperatures. Since the ratio of the relaxation rates at the Cu and Oxygen sites is independent of temperature below T c, indicating that antiferromagnetic spin correlations still remain in the superconducting state and that the superconducting gap is nearly q-independent, the T-dependence of the anisotropy is difficult to understand within the framework of the models of isotropic antiferromagnetic spin fluctuations that have been proposed to analyze the normal state NMR results.